The present invention relates to a light branching/inserting apparatus and a light branching apparatus in an optical network using a wavelength division multiplexing (hereinafter referred to as WDM) method. In particular, the present invention relates to a light branching/inserting apparatus and a light branching apparatus which use a wavelength selection filter utilizing acousto-optic effects, as a filter performing light branching and insertion.
The WDM method utilizing a band characteristic of an optical fiber, is a method anticipated for enlarging the transmission capacity and constructing an optical network which is flexible in the input/output of signals. Compared to the conventional method in which light having one kind of wavelength is modulated and transmitted to one fiber at a high speed, if wavelength division multiplexing is performed using this method with the same transmission speed, the information can be increased by the number of wavelength division multiplexing and transmitted. Furthermore even if the signal is transmitted at a low speed, by wavelength division multiplexing, a similar transmission capacity can be obtained as with the conventional method for transmitting a signal of one wavelength at a high speed. With the WDM method, the interval of each wavelength to be multiplexed should be such that the wavelenghts are apart from each other so that each wavelength is not affected by the adjacent wavelength signal. At present, the band of an optical amplifier has been enlarged (to be more than ten nanometers), and a filter on the reception side having a narrow selection zone has been realized. Hence experiments of a WDM transmission system with a wavelength interval of about 1 nm have been reported, and this system is about to be introduced as an actual system.
Furthermore, the recent trend of research is to realize an optical network based on this WDM transmission system. As the optical network, as proposed in for example, Japanese Unexamined Patent Publication No. 4-167634, there can be mentioned a network having an ADM (Add-Drop Multiplexer) function such that not only WDM signals are transmitted from point to a point, but also only a light signal having a certain wavelength among wavelength division multiplexed light signals is selectively transmitted at a junction point referred to as a node provided along the transmission path, and light signals having other wavelengths are received at the node, or another light signal is inserted from the node and transmitted to other nodes. The ADM function has the characteristic in that it can freely branch or insert a signal in the optical state, and is a technique peculiar to the WDM method.
As the conventional light branching/inserting apparatus (hereinafter referred to as an ADM node apparatus), for example, as shown in FIG. 8, there is an apparatus formed by combining two Arrayed Waveguide Gratings (hereinafter referred to as AWG). The AWG is a device having a light-wave mixing function or a light-wave dividing function, and when a wavelength division multiplexed light signal is input to an input port, a light signal wave-divided for each wavelength is output from each port on the output side. On the contrary, when the AWG inputs light having a previously determined wavelength corresponding to each port, a wavelength division multiplexed light signal in which this light is wave-mixed is output on the output side. The ADM node apparatus formed using the AWG, wave-divides the multiplexed light signal for each wavelength with the AWG on a first stage (on the input side), controls the branching, insertion or transmission, respectively, for each wavelength, and wave-mixes again the light signals of each wavelength with the AWG on a second stage (on the output side) to send them to the transmission path. The control of the branching, insertion or transmission of a light signal having an optional wavelength can be realized, for example, by providing a light switch on each output port in the AWG on the first stage, and controlling the switching state.
With such a conventional ADM node apparatus, the transmission light wavelength characteristics of the AWG are designed in advance corresponding to each wavelength of a multiplexed light signal used for the transmission. Moreover, the input wavelength characteristics and the output wavelength characteristics to/from each port in the AWG are determined not optionally but relatively with periodicity. Therefore, it is important as a function of the ADM node apparatus that each port and the wavelengths of the light signals are always controlled precisely.
With the abovementioned conventional ADM node apparatus, as the number of wavelength division multiplexing increases, the control of the wavelength of light signals for each port becomes complex and the node construction becomes complicated. Moreover, in the designing stage, since the wavelength and the largest number of the wavelengths to be used in the ADM node apparatus are determined beforehand, the ADM node apparatus has a defect in that it is difficult to make this correspond to changes in the wavelengths to be used later, to addition of signals, and so on. Furthermore, the conventional ADM node apparatus has a periodic variation to the wavelength with respect to the transmission characteristics. Hence there is the problem in that with a ring network or the like to be used by connecting ADM nodes in multi stages, measures for correcting the light power for each wavelength and the like become necessary by using, for example, an optical attenuator or the like.
It is effective to use an acousto-optic filter for the ADM filter as one device for facilitating control of the wavelength of a light signal for each port. The ADM node construction using an acousto-optic filter is proposed in, for example, Japanese Unexamined Patent Publication No. 9-113855. The abovementioned technique is to suppress interference noise by shifting a frequency of branched light and insertion light, in order to prevent signal deterioration due to the interference of the branched light and the insertion light which occurs when an acousto-optic filter or the like is used for the ADM node.
With the abovementioned ADM node construction however, since complicated work is necessary for changing the branched/inserted wavelength, and for adding signals and so on, and the insertion light the frequency of which is shifted is wave-mixed with the main light signal at each ADM node and transmitted, then as the number of wavelength division multiplexing in the transmission system increases, the control of the wavelength of the light signal becomes complicated for the whole system.
In view of the above situation, it is an object of the present invention to provide a light branching/inserting apparatus and a light branching apparatus which can control the wavelength of the light signal easily and which can branch, insert or transmit a light signal having an optional wavelength and optional number of division multiplexing by using a wavelength selection filter which utilizes acousto-optic effects.
Therefore, according to one aspect of the present invention, there is provided a light branching/inserting apparatus comprising: a branching/inserting device connected to a transmission path through which wavelength division multiplexed light signals are transmitted, which is capable of branching and inserting light signals having at least one wavelength with respect to light signals on the transmission path; a light branching device for receiving and processing light signals branched by the branching/inserting device for each wavelength; and a light inserting device for outputting insertion light to be inserted into the light signals on the transmission path to the branching/inserting device, wherein the branching/inserting device includes a wavelength selection filter which can generate at least one surface acoustic wave of a frequency corresponding to a selection signal, and which has an input port to which a light signal received from the transmission path is input, an output port from which a light signal to be output to the transmission path is output, a branching port connected to the light branching device and an insertion port connected to the light inserting device, said wavelength selection filter applied with the selection signal and branching a light signal having a wavelength corresponding to the frequency of the surface acoustic wave contained in the light signal which is sent to the input port from the transmission path and outputting the branched light signal to the branching port, as well as inserting an insertion light having a wavelength corresponding to the frequency of the surface acoustic wave and which is sent to the insertion port, into the light signal from the transmission path and outputting the light signal to the output port.
Furthermore, according to another aspect, there is provided a light branching/inserting apparatus connected to a transmission path through which wavelength division multiplexed light signals are transmitted, which is capable of branching and inserting light signals having at least one wavelength with respect to light signals on the transmission path, the apparatus comprising: a light branching device for receiving and processing branched light signals for each wavelength; a light inserting device for generating insertion light to be inserted into the light signals on the transmission path, and a wavelength selection filter which can generate at least one surface acoustic wave of a frequency corresponding to a selection signal, and which has an input port to which a light signal received from the transmission path is input, an output port from which a light signal to be output to the transmission path is output, and a branching port connected to the light branching device and an insertion port connected to the light inserting device, and the wavelength selection filter branches a light signal having a wavelength corresponding to the frequency of the surface acoustic wave contained in the light signal sent to the input port from the transmission path and outputs the branched light signal to the branching port, and also inserts insertion light having a wavelength corresponding to the frequency of the surface acoustic wave and which is sent to the insertion port, into the light signal from the transmission path and outputs the light signal to the output port.
With such a construction, a wavelength division multiplexed light signal on the transmission path is sent to the input port in the wavelength selection filter. Surface acoustic waves of a frequency matched to the wavelength of the light signal to be branched or inserted can be generated in this wavelength selection filter, and only a light signal having a wavelength corresponding to the frequency of a surface acoustic wave among each light wavelength contained in the light signal sent to the input port is subjected to polarization conversion by means of acousto-optic effects and output from the branching port, and light signals having other wavelengths are output from the output port. The light signal output from the branching port is sent to the light branching device to be received and processed for each wavelength. Moreover, insertion light generated by the light inserting device is input to the insertion port of the wavelength selection filter, and the insertion light having a wavelength corresponding to the frequency of the surface acoustic wave is subjected to polarization conversion by means of acousto-optic effects, as in the abovementioned case of branching, inserted into the light signal from the input port, and output from the output port to be sent to the transmission path.
Furthermore, according to another aspect of the present invention, there is provided a light branching apparatus connected to a transmission path through which wavelength division multiplexed light signals are transmitted, which is capable of branching light signals having at least one wavelength from amongst the light signals on the transmission path, the apparatus comprising: a light branching device for receiving and processing branched light signals for each wavelength, and a wavelength selection filter which can generate at least one surface acoustic wave of a frequency corresponding to a selection signal, and which has an input port to which a light signal received from the transmission path is input, an output port from which a light signal to be output to the transmission path is output, and a branching port connected to the light branching device, and the wavelength selection filter branches a light signal having a wavelength corresponding to the frequency of the surface acoustic wave contained in the light signal sent to the input port from the transmission path and outputs the branched light signal to the branching port.
This light branching apparatus corresponds to one which does not have the light insertion function of the abovementioned light branching/inserting apparatus.
Moreover, according to yet another aspect of the present invention, there is provided a light branching/inserting apparatus comprising: a branching/inserting device connected to a transmission path through which wavelength division multiplexed light signals are transmitted, which is capable of branching and inserting light signals having at least one wavelength with respect to light signals on the transmission path; a light branching device for receiving and processing light signals branched by the branching/inserting device for each wavelength; and a light inserting device for outputting insertion light to be inserted into the light signals on the transmission path to the branching/inserting device, wherein the branching/inserting device includes; a wavelength selection filter which can generate at least one surface acoustic wave of a frequency corresponding to a selection signal, and which has an input port to which a light signal received from the transmission path is input, an output port from which a light signal to be output to the transmission path is output, and a branching port connected to the light branching means, and a light wave mixing section for mixing insertion light from the light inserting device into light signals output from the output port and outputting the mixed light signal to the transmission path, and the wavelength selection filter is applied with the selection signal and branches a light signal having a wavelength corresponding to the frequency of the surface acoustic wave contained in the light signal which is sent to the input port from the transmission path and outputs the branched light signal to the branching port, and outputs light signals of other wavelengths to the output port.
Moreover, according to yet another aspect there is provided a light branching/inserting apparatus connected to a transmission path through which wavelength division multiplexed light signals are transmitted, which is capable of branching and inserting light signals having at least one wavelength with respect to light signals on the transmission path, the apparatus comprising: a light branching device for receiving and processing branched light signals for each wavelength; a light inserting device for generating insertion light to be inserted into the light signals on the transmission path, a wavelength selection filter which can generate at least one surface acoustic wave of a frequency corresponding to a selection signal, and which has an input port to which a light signal received from the transmission path is input, an output port from which a light signal to be output to the transmission path is output, and a branching port connected to the light branching device, and a light wave mixing device for mixing insertion light from the light inserting device into light signals output from the output port and outputting the mixed light signal to the transmission path, and the wavelength selection filter is applied with the selection signal and branches a light signal having a wavelength corresponding to the frequency of the surface acoustic wave contained in the light signal which is sent to the input port from the transmission path and outputs the branched light signal to the branching port, and outputs light signals of other wavelengths to the output port.
With such a construction, a wavelength division multiplexed light signal on the transmission path is sent to the input port of the wavelength selection filter. Surface acoustic waves of a frequency matched to the wavelength of the light signal to be branched can be generated in this wavelength selection filter, and only a light signal having a wavelength corresponding to a surface acoustic wave among each light wavelength contained in the light signal sent to the input port is subjected to polarization conversion by means of acousto-optic effects and output from the branching port, and light signals having other wavelengths are output from the output port. The light signal output from the branching port is sent to the light branching device to be received and processed for each wavelength. Moreover, the light signal output from the output port is wave-mixed with insertion light from the light inserting device, in a light wave mixing section and then output to the transmission path.
The abovementioned respective aspect may include at least one optical amplifier which amplifies the light signal up to a power level required for transmission. Thereby the apparatus has a function as a linear repeater. Moreover, the apparatus may comprise a dispersion compensation device for compensating for the dispersion characteristics of the transmission path, and a dispersion compensation light amplifying device for compensating for a loss of the light signal power in the dispersion compensation device. Thereby the influence of the dispersion characteristics of the transmission path on the transmission characteristics of the light signal can be compensated for.
Furthermore, the light branching device may include a light wave dividing section for dividing the light signal output from the branching port of the wavelength selection filter according to the received number of wavelengths, a wavelength selecting section in which the selected wavelength is variable, which is provided for each output port of the light wave dividing section and which selects one light wavelength from the wave-divided light signals and outputs the selected light wavelength, and a reception section for receiving and processing a light wavelength selected by the wavelength selecting section. With this construction, the wavelength of the light signals received and processed by the light branching device can be optionally set, and the number of the light signals can be optionally set within a range of the received number of wavelengths (that is, the number of the wavelength selecting sections and reception sections provided in the light branching device).
Furthermore, the light inserting device may include a light source section for generating light corresponding to all wavelengths which are transmittable on the transmission path; a light wave mixing/dividing section for wave-dividing the wavelength division multiplexed light in which each light wavelength from the light source section is wave-mixed according to the number of inserted wavelengths and outputting the wavelength division multiplexed light; an insertion light generating section in which the selected wavelength is variable, which performs modulation for each wavelength division multiplexed light output from the light wave mixing/dividing section and selects and outputs one light wavelength; a light wave mixing section for mixing a light signal of each wavelength output from the insertion light generating section and outputting the wave-mixed light signals; and a light amplifying section for amplifying a light signal output from the light wave mixing section and outputting the amplified light signal to the insertion port of the wavelength selection filter. In addition, preferably the light inserting device includes a dispersion compensation section for compensating for the dispersion characteristics of the transmission path, and a dispersion compensation light amplifying section for compensating for a loss in the light power in the dispersion compensation section.
With such a construction, the light inserting device can output the insertion light corresponding to all the wavelengths which are transmittable on the transmission path, and can optionally set the wavelength and the number of insertion light signals. Moreover, by outputting insertion light in which the dispersion characteristics of the transmission path has been compensated for, the transmission characteristics of the light signals wave-mixed with the insertion light can be improved.
In addition, the light inserting device described above may include a light source control section for sending only a light wavelength which can be inserted, from the light source section to the light wave mixing/dividing section, or further include an insertion light control section for sending only a light signal of a wavelength to be actually inserted, from the insertion light generating section to the light wave mixing/dividing section. Thus, by providing the light source control section and the insertion light control section, then with respect to the insertion light output from the light inserting device, the leaking in of light wavelengths unnecessary for insertion and the occurrence of cross talk light is prevented.
Furthermore, an apparatus using the abovementioned wavelength selection filter preferably includes a monitoring device for respectively measuring a spectrum of the light signal input from the transmission path to the wavelength selection filter and a spectrum of the light signal output from the wavelength selection filter to the transmission path, to monitor the light power for each wavelength. Moreover, the construction may be such that with the selection signal at least one of the frequency or the output power is adjusted according to the monitoring results of the monitoring device, and with the wavelength selection filter, the light signal power of each wavelength output from the output port is controlled to be almost constant according to the selection signal. In addition, the light inserting device may include a power adjusting section for adjusting the light signal power of each wavelength output from the insertion light generating section according to the monitoring results of the monitoring device.
With such a construction, when there is a difference in the light power of each wavelength of the light signal input/output to/from the wavelength selection filter, a selection signal in which the frequency or the output power is adjusted according to the monitoring results of the monitoring device is sent to the wavelength selection filter, and the power of the insertion light is adjusted by the power adjusting section of the light inserting device. In this way the light signal power of each wavelength output from the output port of the wavelength selection filter is controlled to be almost constant.
Furthermore, the selection signal may have a frequency corresponding to an unused light wavelength which is not contained in the light signal input from the transmission path to the wavelength selection filter. By applying such an RF signal to the wavelength selection filter, noise generated in the unused wavelengths can be removed by the wavelength selection filter.
Moreover, with the abovementioned apparatus preferably the construction includes a temperature control device for controlling a device temperature of the wavelength selection filter to be almost constant, or includes a temperature monitoring device for measuring a device temperature of the wavelength selection filter, and the frequency of the selection signal is corrected based on the measurement results of the temperature monitoring device. As described above, by providing a temperature control device or a temperature monitoring device, the fluctuation in the selected wavelength due to the change of the device temperature in the wavelength selection filter can be suppressed.